1. Field of the Invention
The present invention relates generally to a mobile communication system supporting voice and data services, and in particular, to a method and apparatus for efficiently transmitting and receiving status information of a forward channel in a mobile communication system supporting selective transmit diversity.
2. Description of the Related Art
A typical mobile communication system, e.g., a code division multiple access (CDMA) mobile communication system, provides only a voice service. However, with the development of a communication technology and at the demand of users, research has been carried out on a mobile communication system for supporting not only the voice service but also a data service. In order to meet such a demand, a 1×EV-DV (Evolution in Data and Voice) mobile communication system has been recently proposed.
In the 1×EV-DV mobile communication system, mobile stations (MSs) measure status of a forward channel from a base station (BS) and each transmits the measurement information to the base station, and the base station determines a destination mobile station that will receive forward packet data, and transmission parameters with which the forward packet data will be transmitted, based on status measurement information of the forward channel, received from a plurality of mobile stations. A reverse channel quality indicator channel (R-CQICH) serves to transmit forward channel status information. Specifically, the forward channel status information is information indicating a carrier-to-interference ratio (C/I) obtained by measuring a pilot channel transmitted in a forward direction.
FIG. 1 illustrates information transmitted over a reverse channel quality indicator channel (R-CQICH) in a common 1×EV-DV mobile communication system. In FIG. 1, a mobile station measures a C/I value of a pilot signal transmitted over a forward pilot channel every 1.25 ms-slot period, and transmits the C/I measurement value over R-CQICH.
In the case where a base station supports selective transmit diversity (STD), a mobile station receives forward pilot signals from two or more base station antennas. In this case, the mobile station transmits a C/I value for a forward pilot channel of an antenna showing a larger C/I value and best antenna selection information indicating a corresponding antenna, to the base station over R-CQICH.
For example, FIG. 1 illustrates a method of transmitting 1-bit antenna selection information over R-CQICH in an STD system using two antennas. That is, a mobile station transmits 4-bit forward channel status information indicating a C/I measurement value and 1-bit best antenna selection information every slot.
A base station receives 4-bit C/I information and 1-bit best antenna selection information every slot, and then determines (i) whether to transmit packet data to a mobile station that has transmitted the information, (ii) data rate, code rate and modulation scheme, to be used for transmission of the packet data in case the base station transmits to the mobile station.
FIG. 2 illustrates a structure of an R-CQICH transmitter in a general 1×EV-DV mobile communication system. Referring to FIG. 2, a mobile station receives a forward pilot channel signal at a C/I measurer 210. The C/I measurer 210 measures a C/I value for the received forward pilot channel signal, and provides the C/I measurement value to a C/I information generator 220. If STD is used, the C/I measurer 210 measures C/I values for all transmission antennas of a base station.
The C/I information generator 220 generates 1-bit best antenna selection information by comparing the C/I measurement values, and generates 4-bit C/I information by quantizing a C/I measurement value selected from the C/I measurement values. The 4-bit C/I information indicates a C/I measurement value for a best antenna having the highest C/I measurement value, and the 1-bit best antenna selection information indicates the antenna having the highest C/I measurement value. If STD is not used, the best antenna selection information is fixed to 0 or 1.
A mapping relationship among the 4-bit C/I information, the 1-bit best antenna selection information, and a C/I measurement value of a forward pilot channel (F-PICH) is illustrated in Table 1.
TABLE 1C/I measurement value of F-PICHC/I information (a4, a3, a2, a1, a0)NULL (Below −14.5 dB, or MS not)x0000−14.5 to −13.1 dBx0001−13.1 to −11.6 dBx0010−11.6 to −10.2 dBx0011−10.2 to −8.8 dBx0100 −8.8 to −7.4 dBx0101 −7.4 to −5.9 dBx0110 −5.9 to −4.5 dBx0111 −4.5 to −3.1 dBx1000 −3.1 to −1.6 dBx1001 −1.6 to −0.2 dBx1010 −0.2 to 1.2 dBx1011   1.2 to 2.6 dBx1100   2.6 to 4.1 dBx1101   4.1 to 5.5 dBx1110Above 5.5 dBx1111
In Table 1, one bit in a position a4 represented by x out of the 5-bit information represents best antenna selection information and is used to indicate a transmission antenna having a highest C/I measurement value.
The 4-bit C/I information and 1-bit best antenna selection information generated by the C/I information generator 220 are channel-encoded by a block encoder 230 using a prescribed code rate based on the number of input bits, e.g., a code rate of (12,5). 12 encoded symbols output from the block encoder 230 are Walsh-covered by a Walsh coverer 240 with a prescribed Walsh cover code. The Walsh cover code has a length of 8 bits and is determined by a 3-bit best sector indicator (BSI). The BSI is to indicate a base station (or sector in the case of sector-type base stations) having best forward channel status in an active set, a list of base stations (or sectors in the case of sector-type base stations) with which the mobile station is communicating.
A 96-bit Walsh-covered sequence output from the Walsh coverer 240 is spread by a Walsh spreader 250 with a Walsh code assigned to R-CQICH, and then transmitted to a base station.
In FIG. 2, the 4-bit C/I information and 1-bit best antenna selection information are transmitted every 1.25 ms slot of a reverse channel. When the CQICH is formed in this way, a mobile station using STD must transmit a total of 5 bits in order to report forward channel status to a base station every slot. This amounts to 4,000 bps (=5 bits/1.25 ms) before encoding, cover and spreading is considered, and in a 1×EV-DV system, all mobile stations desiring to receive a packet data service must transmit the information. However, the C/I information transmission method stated above causes a large reduction in reverse transmission capacity by a base station.